This invention relates to article inspection systems and, particularly, to vision systems for inspecting work pieces such as plastic molded closure caps for containers and the like.
During the manufacture of many parts such as plastic molded closure caps, a number of defects in the closure cap may exist which should cause the cap to be rejected. Commonly, closure caps of this type have a liner inserted therein against the inner surface of an end wall of the cap. Typically, the cap has a skirt projecting annularly from the peripheral rim of the end wall and the skirt may include a closure or sealing mechanism such as threads for cooperation with mating threads around the neck of a bottle, container or the like. Examples of defects in such closure caps include a liner which is positioned off center within the closure, a missing liner, a malformed liner (commonly referred to as a xe2x80x9cmoon-cutxe2x80x9d liner), a cap which is asymmetric or off-round, a cap having an edge broken or flashing on the edge from extraneous plastic material, a pull tab defect on the liner or other similar problems. Such flaws or defects are sometimes produced during the manufacturing process and/or as a result of contamination or damage after manufacture, but prior to the filling of the container.
Machine vision systems represent one technology for acquiring or sensing an image of at least a selected portion of a work piece, such as a cap as previously described, through an electronic sensor or camera. The image generated by the camera is then analyzed by a computer program for one or more of the above-described defects. Vision systems are commonly used to determine the existence of any marks or defects in the image of the cap and the acceptability of any such marks or defects by use of a vision computer as described.
While human vision may out perform its automatic equivalent in the ability to analyze very complex, everyday scenes, when it comes to repeated tasks, such as the inspection of plastic molded caps over and over again, a human observer understandably tires, loses concentration and makes mistakes. Machine vision inspection of such articles is known to provide some important advantages, including sophisticated image processing/analysis, repeatable performance, image acquisition for diagnosis and set up, ability to inspect a variety of articles in large tolerance and required part placement. Moreover, at inspection rates of up to 1600 parts per minute or more, each part or cap spends on the order of 33 milliseconds at an inspection station. At such speeds, only a machine vision system is fast enough to reliably and repeatedly inspect such articles.
While known vision systems have the above-described advantages for inspecting articles such as plastic molded caps and the like, they do have specific and significant limitations. Vision systems typically rely on television or video cameras to image the article to be inspected and detect any flaws. The resolution of the camera, or its ability to detect a flaw, is directly related to its ability to capture an accurate and reliable image of each individual cap, article or similar item. Typically, plastic molded caps, for example, are manufactured by the tens of thousands and each individual cap must be inspected by the vision system for quality control purposes. The large volume of caps are typically gathered in an accumulated mass and, at best, are similarly oriented on a flat surface. For accurate vision inspection and detection of flaws, the vision system must be able to precisely and accurately produce an image of each individual cap without interference from the surrounding environment or other caps.
Furthermore, inspection rates required of such systems mandate that the individual images be serially produced, analyzed and acted upon accordingly for each individual cap, once again without interference, for accurate detection of relatively small flaws or problems.
Additionally, plastic molded caps, like most objects being inspected by vision systems, are three-dimensional and have a variety of features and geometries that must be captured by the vision system for analysis. Because of the limited field of view of standard cameras and known vision systems, typically multiple cameras oriented at different angles and having differing fields of view of the work piece or plastic molded cap are utilized to obtain the required information for proper analysis. Utilizing multiple cameras to obtain the required views and information for analysis is both expensive from a hardware equipment standpoint as well as complex and cumbersome from an analysis and vision systems software perspective. For example, one type of plastic molded cap includes a liner having a tab which is folded under between the liner and the adjacent end wall of the cap. Ultimately, the liner is intended to be adhered to the upper rim on the mouth of the container so that when the consumer initially opens the container by unscrewing the cap, the liner remains on the mouth of the cap. The user then would lift the liner from the container by pulling on the now exposed tab. However, during the manufacture of the cap and liner combination, occasionally, the tab is not properly folded under between the end wall and the liner and is presented as a standing tab projecting upwardly from the liner typically along the threaded region of the skirt of the cap. Alternatively, the tab may be missing entirely from the liner.
Other possible defects with plastic molded caps of this type include excess glue or foreign matter on the threads of the skirt of the cap, the top edge of the skirt when the cap is inverted may be malformed from a xe2x80x9cshort shotxe2x80x9d during the injection molding process or other similar defects which must be identified by a vision system. Such defects are difficult if not impossible to accurately assess in a timely and efficient manner with a standard single camera system due to the orientation, geometry, and three-dimensional configuration of the plastic molded cap and liner combination.
This invention provides a machine vision inspection system for inspecting work pieces such as caps and other articles and an associated method for doing so which overcomes the above-stated and other limitations with known systems/methods.
In a presently preferred embodiment, this invention is an inspection system for inspecting each of a series of serially fed work pieces in a stream of work pieces, such as plastic molded caps or the like. The system includes a feed conveyor to serially feed the caps or work pieces, each of which is typically in contact with adjacent work pieces on the feed conveyor in an accumulated mass or the like. The feed conveyor advances the caps to an inspection ramp or platform which in a presently preferred embodiment is inclined between 35xc2x0 and 50xc2x0, and most preferably at 40xc2x0 with respect to a horizontal plane. The inspection ramp has a reduced friction upper surface upon which the caps or other articles advance downwardly from a top end of the inspection ramp toward a bottom end. An optional discharge conveyor is located at the bottom end of the inspection ramp to receive and discharge each of the caps for collection, packaging and/or further processing.
Advantageously, the inspection ramp is inclined so that as the caps which are in contact with one another and therefore difficult for a vision system to accurately inspect and discriminate at a top end of the ramp advance by gravity along the reduced friction surface through an inspection station located between the top and bottom ends of the ramp. The incline of the ramp produces a separation distance between each of the caps so that each cap can be individually and accurately inspected at the inspection station for defects or the like. Preferably, a pair of spaced guide rails are positioned on the lateral sides of the caps to provide for accurate lateral positioning of the caps with respect to the inspection station on the ramp.
The inspection station in a presently preferred embodiment includes an inspection window in the ramp, an infrared or other color LED strobe light source and a camera. The light source is preferably located on a back side of the inspection ramp to project light through the inspection window to back-light and illuminate each of the caps as they pass above the window. Back-lighting of the caps avoids the glare commonly generated from foil caps and liners and likewise offers a contrasting image even with white caps and white liners for accurate imaging. The inspection window and light source are preferably aligned with the camera which is located on a top side of the inspection ramp and oriented generally perpendicularly with respect to the inspection ramp at the inspection station. Preferably, the positioning of the light source, inspection window and camera provides for a full and complete image of the cap for accurate resolution and detection of possible defects in the cap.
One advantageous aspect of this invention is the ability for the inspection system to accurately and efficiently capture an image of a three-dimensional object such as a plastic molded cap while utilizing fewer and in most cases, only a single camera. Accurate images of various three-dimensional aspects of the work piece, such as an internal thread region on the skirt of the inverted cap passing on the inspection platform past the camera are accurately imaged by a single camera through the use of a reflective device which, in one embodiment of this invention is a cone. The cone is positioned within a viewing cylinder in the field of view of the camera to deflect a portion of the image of the cap, such as the thread region on the inner surface of the skirt of the cap, for accurate imaging by the camera. Additionally, portions of the field of view of the camera are not reflected by the cone so that additional aspects of the work piece can likewise by imaged and assessed by the vision system.
The system also includes a processing unit such as a computer or the like operably coupled to the camera to analyze the images of the caps generated by the camera with respect to predetermined quality control standards. For example, specific defects as described herein above, if detected by the camera, are outside of the predetermined quality control standards utilized by the computer to analyze each of the images generated by the camera. If a particular cap fails the analysis, a rejection mechanism, typically an air jet or the like, is coupled to the processing unit to receive a control signal from the processing unit and, if such a signal is received by the air jet or other rejection mechanism, the identified cap is then removed from the stream by the air jet or other rejection mechanism. The rejected cap is then discarded, analyzed or recycled as is appropriate.
This invention in a presently preferred embodiment overcomes the above-described disadvantages of known vision inspection systems by accurately and reliably providing a separation distance between each of the serially fed work pieces, articles, caps or the like to be inspected as a result of the inspection ramp and the inclination thereof. Further, the processing or inspection rate is not diminished as a result of the orientation and configuration of the inspection ramp, reflective cone and inspection station components thereby providing inspection rates as high as 1600 per minute or more depending upon the size of the items being inspected and other operational requirements.
Furthermore, heretofore difficult to inspect or analyze work pieces such as the intricate geometries of plastic molded caps and liners are reliably and effectively illuminated for accurate imaging and detection by a single camera and associated processing unit because of the opportunity for back lighting the cap with an infrared or other color LED strobe light source in combination with the reflective cone.